This invention relates to methods utilizing nuclear magnetic resonance (NMR) techniques for studying an object. In particular, this invention relates to two- and three-dimensional rapid NMR data acquisition schemes, useful in but not limited to NMR imaging.
By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons and/or neutrons. Due to the spin of the protons and the neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field, B.sub.0, a greater number of nuclear magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of magnetic field B.sub.0, the magnetic moments precess about the field axis at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, .omega., also referred to as the Larmor frequency, is given by the equation .omega.=.gamma.B, in which .gamma. is the gyromagnetic ratio which is constant for each NMR isotope and wherein B is the magnetic field (including B.sub.0) acting upon the nuclear spins. It will be thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned.
The orientation of magnetization M, normally directed along the magnetic field B.sub.0, may be perturbed by the application of magnetic fields oscillating at the Larmor frequency. Typically, such magnetic fields designated B.sub.1 are applied orthogonally to the direction of the static magnetic field by means of a radio frequency (RF) pulse through coils connected to a radio-frequency-transmitting apparatus. The effect of field B.sub.1 is to rotate magnetization M about the direction of the B.sub.1 field. This may be best visualized if the motion of magnetization M due to the application of RF pulses is considered in a Cartesian coordinate system which rotates (rotating frame) at a frequency substantially equal to the resonant frequency .omega. about the main magnetic field B.sub.0 in the same direction in which the magnetization M precesses. In this case, the B.sub.0 field is chosen to be directed in the positive direction of the Z-axis, which, in the rotating frame, is designated Z' to distinguish it from the fixed-coordinate system. Similarly, the X- and Y-axes are designated X' and Y'. Bearing this in mind, the effect of an RF pulse, then, is to rotate magnetization M, for example, from its direction along the positive Z' axis toward the transverse plane defined by the X' and Y' axes. An RF pulse having either sufficient magnitude or duration to rotate (flip) magnetization M into the transverse plane (i.e., 90.degree. from the direction of the B.sub.0 field) is conveniently referred to as a 90.degree. RF pulse. Similarly (in the case of a rectangular pulse), if either the magnitude or the duration of an RF pulse is selected to be twice that of a 90.degree. pulse, magnetization M will change direction from the positive Z' axis to the negative Z' axis. This kind of an RF pulse is referred to as a 180.degree. RF pulse, or for obvious reasons, as an inverting pulse. It should be noted that a 90.degree. or a 180.degree. RF pulse (provided it is applied orthogonal to M) will rotate magnetization M through the corresponding number of degrees from any initial direction of magnetization M. It should be further noted that an NMR signal will only be observed if magnetization M has a net transverse component (perpendicular to B.sub.0) in the X'-Y' (transverse) plane. A 90.degree. RF pulse produces maximum net transverse magnetization in the transverse plane since all the magnetization M is in that plane, while a 180.degree. RF pulse does not produce any transverse magnetization.
RF pulses may be selective or nonselective. Selective pulses are typically modulated to have a predetermined frequency content so as to excite nuclear spins situated in preselected regions of the sample having magnetic-field strengths as predicted by the Larmor equation. The selective pulses are applied in the presence of localizing magnetic-field gradients. Nonselective pulses generally affect all of the nuclear spins situated within the field of the RF pulse transmitter coil and are typically applied in the absence of localizing magnetic-field gradients.
There are two exponential time constants associated with longitudinal and transverse magnetizations. The time constants characterize the rate of return to equilibrium of these magnetization components following the application of perturbing RF pulses. The first time constant is referred to as the spin-lattice relaxation time (T.sub.1) and is the constant for the longitudinal magnetization to return to its equilibrium value. For biological tissue, T.sub.1 values range between 200 milliseconds and 1 second. A typical value is about 400 milliseconds. Spin-spin relaxation time (T.sub.2) is the constant for the transverse magnetization to return to its equilibrium value in a perfectly homogeneous field B.sub.0. T.sub.2 is always less than T.sub.1 and in biological tissue, the range is between about 50 to 150 milliseconds. In fields having inhomogeneities, the time constant for transverse magnetization is governed by a constant denoted T.sub.2 *, with T.sub.2 * being less than T.sub.2.
There remains to be considered the use of magnetic-field gradients to encode spatial information (used to reconstruct images, for example) into NMR signals. Typically, three such gradients are necessary: EQU G.sub.x (t)=.differential.B.sub.0 /.differential.x, EQU G.sub.y (t)=.differential.B.sub.0 /.differential.y, and EQU G.sub.z (t)=.differential.B.sub.0 /.differential.z.
The G.sub.x, G.sub.y, and G.sub.z gradients are constant throughout the imaging slice, but their magnitudes are typically time dependent. The magnetic fields associated with the gradients are denoted, respectively, b.sub.x, b.sub.y, and b.sub.z, wherein EQU b.sub.x =G.sub.x (t)x, EQU b.sub.y =G.sub.y (t)y, EQU b.sub.z =G.sub.z (t)z,
within the volume.
In the past, the NMR phenomenon has been utilized by structural chemists to study in vitro the molecular structure of organic molecules. More recently, NMR has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects, for example. Such images depicting nuclear-spin distribution (typically protons associated with water in tissue), spin lattice (T.sub.1), and/or spin-spin (T.sub.2) relaxation constants are believed to be of medical diagnostic value in determining the state of health of tissue in the region examined. Imaging data for reconstructing NMR images is collected by subjecting the sample to pulse sequences comprised of magnetic-field gradients and RF pulses. A drawback associated with some data acquisition schemes is the prohibitively long scan time needed to acquire the necessary data. Efforts to reduce the total acquisition time by lowering the repetition time (T.sub.r) between pulse sequences is limited by the finite relaxation times which, in biological tissues, typically range from 200-600 millisec. Stated differently, the nuclear spins are progressively saturated as the repetition time is shortened. Saturation is a non-equilibrium state in which equal numbers of nuclear spins are aligned against and with magnetic field B.sub.0, so that there is no net magnetization M. Thus, it will be recognized that under conditions of saturation, nuclear spins cannot be excited to produce an NMR signal. It is, therefore, a principal object of the present invention to provide an NMR data acquisition scheme which is capable of rapidly collecting the data necessary for reconstructing NMR images, for example.